Let $X$ and $Y$ be sets: $X = \{15,9,10\}$ $Y = \{4,10,11,5\}$ What is the set $X \cup Y$ ?
Answer: Remember that $\cup$ refers to the union of sets. The union of two sets $X$ and $Y$ is the collection of elements which are in $X$ or in $Y$ or in both $X$ and $Y$ The members of a set must be unique, and the order doesn't matter. $X \cup Y = \{4,5,9,10,11,15\}$